The only 'issue' I had with buckwheat size coal is that the larger coal seems to have less friction on the grate, the fire and the ashes slid off the grate when I could'nt leave it alone and I poked at the fire on the grate,,,,, and the whole fire and ash slid off into the ashpan...

So after that I didn't mess with the fire or the ashes, and it worked fine... I'd give the Buck coal a try, mixed at first then see if your stove likes it 100%. You may need to change your feed and air settings.